In magnonic devices, wave properties and magnetism are combined on a nano-scale, e.g., spin waves (also referred to as magnons) are formed and applied in such devices. Spin waves may be considered as states of magnetization that propagate as a wave in a suitable material, e.g., a ferromagnetic material shaped and isolated so as to form a magnetic spin wave waveguide element.
A magnonic crystal, as known in the art, is a component formed by a medium allowing spin wave transmission, in which a lattice is formed by periodic modulation of material properties of the medium and/or of the geometry. The modulation length of such a lattice may, for example, be of the order of the wavelength of the transmitted spin waves. For example, a magnonic crystal may comprise a material or materials configured such that a periodic modulation of magnetic parameters is achieved, e.g., by directly engineering the material composition or by modulating a geometric configuration of the material. The spin wave transmission spectrum of such artificial lattices may typically comprise band gaps, e.g., frequency ranges in which propagation of the spin wave is suppressed, inhibited, or prohibited. For example, the periodic modulation of magnetic parameters in a magnonic crystal may give rise to artificially configured band structures, thus providing good control over the spin wave transmission spectrum.
An example of a magnonic crystal may comprise a one-dimensional diffraction grating created in a ferromagnetic waveguide for spin waves. For example, such a waveguide may be formed by a strip of ferromagnetic material. The grating may modulate the internal magnetic field periodically in the direction of propagation along the waveguide structure to achieve a predetermined band structure.
The periodic nature of such a lattice may be conventionally predetermined when the device is manufactured. However, it may be advantageous for various applications to provide a magnonic crystal having tunable transmission spectra, e.g., in which the band gap structure may be adjusted during operation of the device.
The present disclosure relates particularly to tunable magnonic crystals and their applications. This tunability should be interpreted as not only allowing a configuration of the spectral band structure by careful design and manufacture of the device, but allowing a reconfiguration of the spectral properties, e.g., the pass band frequencies, during operation of the device. Thus, band structures in such a tunable magnonic crystal device may be advantageously reprogrammed during operation. Unlike for similar periodic artificial structures for manipulating the flow of plasmons, elastic waves, or acoustic waves, magnonic crystals have a band structure that not only depends on a periodic patterning of the structure, but is strongly dependent on a spatial configuration of magnetization vectors in the crystal.
One possible application of a tunable magnonic crystal could be in radio frequency (RF) filters. Conventional RF filters are widely used in mobile phones, Bluetooth, satellite navigation and communication, and wireless local area networks (WLAN). For example, the number of RF filters used in a single mobile phone may range from three to seven filters. However, integrated on-chip solutions for RF filters are currently mainly based on LC circuits. Further, conventional solutions may use surface-mounted standalone devices, such as SAW or BAW filters. However, integrated, as well as standalone, devices are relatively large with respect to typical integrated circuits. The size of these devices is typically of the order of a square millimeter, while integrated LC filters are typically limited by the inductor size, which may be of the order of at least 104 μm2 per inductor. SAW and BAW filters are furthermore difficult to integrate on-chip, and difficult to scale without increased losses. SAW filters are moreover limited in performance and maximum frequency. Thus, it may be advantageous to replace such RF filter components by an on-chip integrated component of small size. Such integrated component may also have a small size while providing a good performance and a large frequency range. Such a filter may also be tunable, e.g., to be able to compensate for passband drift.